1. Field of the Invention
The invention relates to a method of determining moisture with a capacitive moisture-sensitive element, and an apparatus for carrying out the method.
2. Description of Prior Art
Such apparatuses and methods are advantageously used in the heating, ventilation and air-conditioning art (HVAC) for buildings for determining the air humidity in a room or in a device for supplying or discharging air.
Capacitive moisture measuring elements deliver a capacitance value in dependence on the air humidity in the area around the measuring element. That capacitance value can be measured by means of an electronic evaluation system. A current moisture value is ascertained with the measured capacitance value, using further parameters such as temperature and comparative parameters.
Capacitive moisture measuring elements are available on the market in various different forms, and by way of example reference will be made here to ‘HS11100’ from Humirel, ‘MiniCap2’ from Panametrics, ‘H5000’ from Gefran, ‘Hygromer C-94’ from Rotronic and ‘HC1000’ from E+E Elektronik. The manufacturers of moisture measuring elements generally also propose circuits, by means of which air humidity can be measured. In many cases an oscillator circuit with a generally known multivibrator ‘555’ is proposed for use of the specified moisture measuring elements.
U.S. Pat. No. 5,844,138 also discloses a device of that kind in which a capacitive moisture sensor is part of an oscillator. The frequency of the oscillator is dependent on the condition of the moisture sensor and thus the moisture level.
In known devices for measuring air humidity, the capacitive moisture sensor used is considered as an ideal capacitor involving a variable capacitance. Experience has shown that this simplification, for certain uses, gives rise to unacceptably large deviations between an air humidity value ascertained in the known manner and the air humidity value which is actually present. In particular mention should be made of the fact that the properties of individual examples in the case of capacitive moisture measuring elements of the same kind can suffer from relatively severe scatter.